3D layer-by-layer photonic crystals possess a full photonic band gap. Simulation of 3D layer-by-layer photonic crystals can optimize the parameters of the photonic crystals to get useful photonic band gap by solving Maxwell’s equations using the plane-wave-based transfer-matrix method. The relations between the parameters (rod pitch a, rod width w, rod thickness h and rod refractive index n) and the photonic band gap have been simulated. We also have fabricated a 3D layer-by-layer photonic crystal with femtosecond laser microfabrication technique through two-photon-absorption photopolymerization of resin. Its reflection spectra have been detected which agree with the simulation result.